Premier pôle paléomagnétique,d'âge Moscovien contraint par un test du pli,obtenu dans le bassin d'Illizi (Craton saharien,Algérie)First palaeomagnetic pole,of Moscovian age constrained by a fold test,in the Illizi Basin (Saharan Craton,Algeria)

Palaeomagnetic study, carried out in the Moscovian (～305 Ma) formation in the Edjeleh anticline, shows the existence of three magnetisation components. Two of them are probably Cenozoic and Permian remagnetisations. The third component determined by both well defined ChRMs and remagnetisation circles analysis passes the fold test. Because the folding started before or during the Stephano-Autunian, this third component is the primary magnetisation. Its palaeomagnetic pole (28.3°S, 58.9°E), close to other poles from the Saharan platform obtained from neighbouring periods but without palaeomagnetic tests, confirms the age of these last data. To cite this article: B. Bayou et al., C. R. Geoscience 334 (2002) 81–87.     

Âge 40K/40Ar,Carbonifère inférieur,du magmatisme basique filonien du synclinal paléozoïque de Tin Serririne,Sud-Est du Hoggar (Algérie)

Palaeozoic formations of the Tassilis Oua-n-Ahaggar (southeastern Hoggar) include magmatic rocks in the Tin Serririne syncline. Slight contact metamorphism of the overlying bed and studies of anisotropy of magnetic susceptibility of these rocks show that the latter correspond to sills and NW–SE or north–south dykes. ⁴⁰K/⁴⁰Ar dating of separated feldspars and whole rock for one sample and of whole rock for two other samples give a mean age of $347.6\pm 16.2\text{Ma}$ (at the 2-σ level), thus corresponding to a Lower Carboniferous (Tournaisian) age. Taking into account both the age of this magmatism and the stratigraphic and structural data for this region suggests that dolerites were emplaced within distensive zones that are related to the reactivation of Panafrican faults. To cite this article: H. Djellit et al., C. R. Geoscience 338 (2006).     

Late Panafrican evolution of the main Hoggar fault zones: Implications of magnetic fabric study in the In Telloukh pluton (Tin Serririne basin, Algeria)

The Late Panafrican evolution of the Hoggar shield is characterized by emplacement of magmatic intrusions and by occurrence of major shear zones separating different terranes. In Telloukh granite is close to the In Guezzam faults (western border of the Tin Serririne basin). Analysis of its visible and magnetic fabrics suggests an emplacement mode and deformation that are not related to the In Guezzam faults, but most likely to a N–S compression, an event not yet identified. Dioritic dykes crosscutting the granite have a very different magnetic fabric, which is related on the contrary to dextral strike-slip movements along the In Guezzam faults. In both cases, no visible fabric can be correlated with the magnetic fabric, which has been likely acquired during late magmatic stages. This magnetic fabric was not significantly affected by the tectonic events that took place after entire crystallization of the magma. The In Guezzam faults and the major 7°30 and 4°50 shear zones are close to intrusions such as In Telloukh dykes and the Alous En Tides and Tesnou plutons where quite similar magnetic fabrics are observed, all related with dextral strike-slip movements along these structures.     

New structural implications for the central Sahara (Algeria), from the revisited Upper Carboniferous “Hassi Bachir” Formation: Paleomagnetic constraints

Paleomagnetic investigations of the folded Upper Namurian–Lower Moscovian “Hassi Bachir” Formation cropping out in the “Ahnet” basin (Central Sahara, Algeria) yield two magnetic components. A pre-folding primary magnetization (D = 136.1°, I = 22.0°, k = 217, α₉₅ = 2.6°) enables us to define a paleomagnetic pole (32.8°S, 55.7°E, K = 328 and A₉₅ = 2.0°) which better constrains a paleopole that was determined by Daly and Irving [Daly, L., Irving, E., 1983. Paléomagnétisme des roches carbonifères du Sahara central; analyse des aimantations juxtaposées; configurations de la Pangée. Ann. Geophys. 1, 207–216] for the same formation. A secondary component consists in a synfolding remagnetization and shows that post-Permian tectonics account for at least about half of the total folding in the studied area. This indicates that Mesozoic folding noted 150 km to the West in the Reggane basin [Smith, B., Derder, M.E.M., Henry, B., Bayou, B., Amenna, M., Djellit, H., Yelles, A.K., Garces, M., Beamud, E., Callot, J.P., Eschard, R., Chambers, A., Aifa, T., Ait Ouali, R., Gandriche, H., 2006. Relative importance of the Hercynian and post-Jurassic tectonic phases in the Saharan platform: a palaeomagnetic study of Jurassic sills in the Reggane basin (Algeria). Geophys. J. Int. 167, 380–396] is not local and affected at least the entire north-western part of the Hoggar area. This reconfirms that the folding of the Paleozoic cover in the Sahara platform should not be restricted to the Hercynian orogeny.     

Seismic hazard assessment in the megacity of Blida (Algeria) and its surrounding regions using parametric-historic procedure

The region of Blida is characterized by a relatively high seismic activity, pointed especially during the past two centuries. Indeed, it experienced a significant number of destructive earthquakes such as the earthquakes of March 2, 1825 and January 2, 1867, with intensity of X and IX, respectively. This study aims to investigate potential seismic hazard in Blida city and its surrounding regions. For this purpose, a typical seismic catalog was compiled using historical macroseismic events that occurred over a period of a few hundred years, and the recent instrumental seismicity dating back to 1900. The parametric-historic procedure introduced by Kijko and Graham (1998, 1999) was applied to assess seismic hazard in the study region. It is adapted to deal with incomplete catalogs and does not use any subjective delineation of active seismic zones. Because of the lack of recorded strong motion data, three ground prediction models have been considered, as they seem the most adapted to the seismicity of the study region. Results are presented as peak ground acceleration (PGA) seismic hazard maps, showing expected peak accelerations with 10% probability of exceedance in 50-year period. As the most significant result, hot spot regions with high PGA values are mapped. For example, a PGA of 0.44 g has been found in a small geographical area centered on Blida city.     

Sur la présence d'une série molassique (de type série pourprée) au Sud-Est de l'Ahaggar (In Guezzam,Ahaggar, Algérie)Presence of molassic series (‘Série pourprée’ type) in the Southeast of the Ahaggar (In Guezzam,Ahaggar, Algeria)

In the western Ahaggar shield, the transition between the Precambrian and the Ordovician units is characterised by thick volcano-sedimentary series (‘Série pourprée’ of the Ahnet). This series, in part of Cambrian age, results from the demolition of the Panafrican belt. Similar series were known in grabens located between the West African craton and the Ahaggar, from the ‘Adrar des Iforas’, in the south, to the Ougarta belt, in the north. We describe in this study a new formation identical to the ‘Série pourprée’ of the Ahnet, but cropping out in the far Southeast of the Ahaggar (In Guezzam). This new datum improves the Panafrican belt configuration. To cite this article: H. Djellit et al., C. R. Geoscience 334 (2002) 789–794.     

Plio-Quaternary reactivation of the Neogene margin off NW Algiers, Algeria: The Khayr al Din bank

The Algiers region, northern Algeria, is known to be seismically active, with recurrent large (M>6) earthquakes. Because of the lack of high-resolution bathymetry, the offshore structures remained for a long time poorly known. Thanks to a new marine data base (MARADJA 2003 cruise), the offshore part of the margin is accurately mapped, and new active and recent structures are described. West of the bay of Algiers, the margin enlarges, forming the Khayr al Din bank, interpreted as a tilted block of the passive margin born during the opening of the Algero-Provençal basin. At the slope break, a 80 km-long fault-tip Quaternary fold, namely the Khayr al Din fault, extends at the foot of the margin off NW Algiers and represents the largest active structure of the coastal area, together with the Sahel anticline. We also map for the first time a set of overlapping, en echelon active folds in the upper part of the Khayr al Din bank, located off previously known active structures on land. Most of these faults represent actually a threat for the Algiers region in terms of seismic hazard but also geological hazards, such as tsunamis, as most of them depicts significant dimensions and slip rates. The highest long-term horizontal shortening rate is found on the Khayr al Din fault and is estimated at 0.5 ± 0.1 mm/yr, with a maximal magnitude of 7.3, which provides one of the highest seismogenic potential in the region.A new tectonic framework for the Algiers region is proposed, in which the main south-dipping offshore structure, of opposite vergence relative to most thrusts on land, appears to be nowadays the main driving fault system, as also found further east in the Boumerdès (M 6.8) 2003 rupture zone. The overall apparent pop-up structure of the recent and active faults may result from a progressive migration of the plate limit from the Late Miocene, north-dipping suture zone on land, to the Quaternary, south-dipping main Khayr al Din fault at sea, suggesting a process of subduction inception.